Email, text messaging, and other forms of digital messages are part of a growing trend in electronically-exchangeable and -storable written communication. Message privacy can often be a concern, but users frequently rely on the relative anonymity provided by message traffic volume to decrease the likelihood of compromise. At best, this reliance provides a false sense of security.
Message content is decided by the author; however, the handling of and access to a message falls outside the control of the sender once the message has been transmitted. A message sender is wholly at the mercy, or whim, of intermediaries, which effect message exchange and there are no guarantees or assurances that privacy will be afforded. Moreover, intermediaries lack control over the actions of third parties acting from outside of authorized communication channel. As a result, privacy over message content cannot be ensured completely unless the sender takes specific precautions to protect their privacy.
Moreover, privacy is particularly important to protect messages while in transit, which are susceptible to attack by parties acting from within and from outside of the communication channel. Targeted and profiling attacks are but two examples. In a targeted attack, a specific sender's messages are singled-out as targets to be compromised for surreptitiously acquiring information, generally for a covert purpose. In a profiling attack, the messages of a group or population of users are targeted, with no particular significance attached to any individual sender. A profiling attack aims to classify a population of users into categories based on semantic content extracted from messages exchanged. The categories can be used for delivering contextual advertisements or other purposes.
Strong encryption, including symmetric and public key encryption, provides the highest form of privacy attack protection, but at a cost. Symmetric encryption requires that both the sender and recipient of a message have a copy of the same encryption key, which requires advance planning and secure key exchange. Similarly, public key encryption requires a support infrastructure to register and issue a public encryption key to requesting parties. Thus, privacy is available, but at the significant expense of key exchange and management mechanisms. Furthermore, the use of symmetric or public-key encryption is easily detected and may draw unwanted attention to encrypted communications.
Steganography provides an undetectable form of privacy protection. Steganography hides or embeds information within a block of host data where host data alterations are imperceptible, such as by encoding audio data in frequency ranges that are humanly inaudible. Lexical steganography hides blocks of text at the lexical or word level through data hiding. In one form of lexical steganography, synonyms are substituted for words having equivalent meaning. The individual words signify bit values, which have relevance when decoded and combined. However, lexical steganography produces ciphertext that is significantly larger than the source message and thus incurs high communication costs. Over time, this high communication cost may also expose the fact that steganography is being used.
Therefore, there is a need for providing privacy to messages in a way that is both efficient and undetectable.